1. Field of the Invention
The present invention relates to a composition for film formation which can be formed into a porous film that excels in dielectric properties, adhesion, film consistency and mechanical strength, and has reduced absorption; a porous film and a method for forming the same; and a semiconductor device which contains the porous film inside.
2. Description of Related Art
In the fabrication of semiconductor integrated circuits, as the circuits are packed tighter, an increase in interconnection capacitance, which is a parasitic capacitance between metal interconnections, leads to an increase in interconnection delay time thereby hindering the enhancement of the performance of semiconductor circuits. The interconnection delay time is called an RC delay, and correlates to the product of the electric resistance of the metal interconnections and the static capacitance between the interconnections. Therefore, reducing the interconnection delay time requires reducing the resistance of metal interconnections or the interconnection capacitance.
The reduction in the interconnection capacitance can prevent a densely packed semiconductor device from causing an interconnection delay, thereby realizing a smaller and faster semiconductor device with reduced power consumption.
One method for reducing interconnection capacitance is to reduce the relative permittivity (dielectric constant) of an interlevel (interlayer) insulator film disposed between metal interconnections. As such an insulator film with a low relative permittivity, it has been considered to use a porous film instead of a silicon oxide film which has been used conventionally. A porous film can be said to be the only practical film as a material with a relative permittivity of 2.2 or less, and various methods for forming a porous film have been proposed.
A first method for forming a porous film is as follows: a precursor solution of a siloxane polymer containing a thermally unstable organic component is synthesized; then the precursor solution is applied on the substrate to form a coating film; and later, a heat treatment is applied to decompose and volatilize the organic component. The resulting film has a number of micro-pores.
As a second method for forming a porous film, it is known to carry out processing by applying a silica sol solution onto a substrate by coating or using a CVD method so as to form a wet gel; then, the silica sol is subjected to a condensation reaction while restricting volume reduction by precisely controlling the speed of the evaporation of the solvent from the wet gel while using solvent exchange method.
As a third method for forming a porous film, it is known that a silica micro-particle solution is applied on a substrate to form a coating film, and then the coating film is sintered to form a number of micro-pores between silica micro-particles.
As a fourth method, Japanese Patent Provisional Publication No. 2000-44875 proposes a composition for porous film formation, which is characterized as containing (A) a component expressed by (R1)nSi(OR2)4-n, wherein R1 is a univalent organic radical, and m is an integer of 0 to 2; (B) a metal chelate compound; and (C) a compound having a polyalkylene oxide structure.
However, these methods have respective major drawbacks as follows. In the first method for forming a porous film, the synthesis of the precursor solution of the siloxane polymer increases the cost. In addition, the formation of the coating film by coating the precursor solution increases the amount of silanol groups remaining in the coating film, which causes a degassing phenomenon indicating the evaporation of water and the like in the heat treatment process that is conducted later and which also deteriorates the film quality due to the porous film absorbing humidity.
In the second method for forming a porous film, the speed control of the evaporation of the solvent from the wet gel requires a special type of coating device, which increases the cost. In addition, a significant amount of silanol remains on the surface of the micro-pores which must be changed to silyl group because otherwise hygroscopicity is high and the film quality decreases. The process to change silanol to silyl makes the process more complicated. In the case where a wet gel is formed by the CVD process, it is necessary to use a special type of CVD device which is different from the plasma CVD device generally used in the semiconductor process, thereby also increasing the cost.
In the third method for forming a porous film, the resulting film has the micro-pores having a very large diameter, since defined by the accumulation structure of the silica micro-particles. This makes it difficult to set the relative permittivity of the porous film to 2 or below.
In the case of the fourth method, out of the three components (A), (B) and (C), the metal chelate compound (B) is essential so as to increase the compatibility of the components (A) and (C), and to make the thickness of the coating film uniform after being hardened. However, the addition of compound (B) is not preferable because it makes the manufacturing process complicated and increases the cost. Therefore, it is desired to develop a material which enables a homogeneous solution to be formed without addition of a chelate component and the coating film to be flat after being hardened.
Moreover, the following problems exist in all the abovementioned methods. Namely, the mechanical strength of the porous film decreases. In order to maintain sufficient mechanical strength of the film, the pore size should be minute and uniform, and the hole should be distributed uniformly in the membrane.
On the other hand, it is also necessary to decrease the process temperature in order to maintain the stability of the wiring material, and the material is desired which can be completely hardened at a temperature below 450° C., preferably 400° C., and exhibit adequate mechanical strength. Such a material is desired as those which make it possible to form multilayer interconnection at a temperature below the melting point of aluminum, which is used as a wiring material, which prevent copper from being oxidized.
As mentioned above the related materials have several problems: the decrease of the quality of the film during the heat treatment process, and increase of the cost. Moreover, there is a problem of poor characteristics of spreading when the porous film is formed. Furthermore there is a problem that adequate mechanical strength may not be obtained when the related porous film is incorporated as a insulating layer into a multilayer interconnection.
As above there is a problem that the large dielectric constant of the porous film used as an insulator film in the multilayer interconnection of the semiconductor device makes the RC delay in the multilayer interconnection of the semiconductor device increased, and makes it impossible for the performance of the semiconductor device (high speed and low power consumption) to be improved. Furthermore, a porous film with a low mechanical strength deteriorates the reliability of the semiconductor device.